If you're new around here, I suggested you scroll down to the bottom lefthand corner of this page and look under the "Help" section. There you will find the FAQs (frequently asked questions), Newbie Central and Tips (for spellcasting). It would greatly benefit you to read through those things. You could also go to the articles tab at the top and read some of the featured articles. It wouldn't hurt to read some of the stickied forum posts either.
For instance, here is a great post to get started with:
No, you cannot join more than one coven. If you desire to be in more than one coven, you could make more than one account and let the priest/ess of the covens know you have multiple accounts.
If you have more questions let us know :)